Electroluminescent devices have features including wide angle of visibility by virtue of selfluminous nature and lower power consumption. Because of these features, up to now, various inorganic electroluminescent devices using inorganic compounds as luminescent materials and various organic electroluminescent devices using organic compounds as luminescent materials (these organic compounds will be hereinafter referred to as “organic luminescent materials”) have been proposed, and an attempt has been made to put the electroluminescent devices into practical use.
Among others, organic electroluminescent devices can significantly reduce necessary application voltage, as compared with inorganic electroluminescent devices. This has led to active studies on the development of organic electroluminescent devices having higher performance through the development of and an improvement in materials for the organic electroluminescent devices. Studies on the utilization of organic electroluminescent devices as a surface light source has been put forward. At the same time, the development of organic electroluminescent devices capable of emitting various colors has led to studies on the utilization of organic electroluminescent devices as pixels of displays. In a display using organic electroluminescent devices as pixels, a plurality of organic electroluminescent devices are two-dimensionally arranged on an identical plane to form a panel (a display panel), and these devices are driven independently of each other or one another to display a desired image.
FIG. 1 is a schematic diagram illustrating a basic construction of an organic electroluminescent display. As shown in FIG. 1, the organic electroluminescent display includes a light transparent substrate 1 and, stacked on the substrate 1 in the following order, a transparent anode 2, an organic luminescent layer (hereinafter simply referred to as “luminescent layer”) 3, and a cathode 4. A display comprising the transparent anode 2, the luminescent layer 3, and the cathode 4 stacked in that order on a specific substrate 1 is an organic electroluminescent display referred to in the present invention. The position of the anode and the position of the cathode are sometimes reversed. Further, in order to improve the performance, the interposition of a hole transport layer between the anode and the luminescent layer, the interposition of an electron injection layer between the cathode and the luminescent layer, or the interposition of an adhesive layer between the cathode and the luminescent layer or between the electron injection layer and the luminescent layer is sometimes adopted. The luminescent layer is generally formed of one or a plurality of organic luminescent materials. In some cases, however, the luminescent layer is formed of, for example, a mixture composed of an organic luminescent material and a hole transport material and/or an electron injection material.
Further, in the organic electroluminescent display, in general, a surface in a substantially parallel positional relationship with the main surface of the luminescent layer serves as a light outgoing surface, and, in the pair of electrodes (anode and cathode) constituting the organic electroluminescent device, the electrode (=anode) located on the light outgoing surface side is formed of a transparent or translucent thin film (hereinafter often referred to as “transparent electrode”) for light take-out efficiency improvement purposes or for reasons of construction of a surface emitting device. On the other hand, the electrode (=cathode) located opposite to the light outgoing surface is formed of a specific metallic thin film (a thin film of a metal, an alloy, a mixed metal or the like).
The above organic electroluminescent display also involves several problems to be solved. One of the problems is low effective light take-out efficiency. Even in the case of an organic luminescent material which exhibits a considerably high internal quantum efficiency in the luminescence, since the refractive index of the substrate is high, the critical angle at which the light emitted from the electroluminescent layer can be radiated to the outside of the electroluminescent display is small. Therefore, as shown in FIG. 1, a considerably large proportion of light emitted from the electroluminescent layer cannot be radiated from within the substrate to the outside of the electroluminescent display and is propagated in the facial direction while undergoing multiple reflection before radiation to the outside of the electroluminescent display. Further, in many cases, the wavelength of light emitted from the electroluminescent layer is likely to be absorbed in the substrate. Therefore, multiple reflection of the light within the substrate is disadvantageous from the viewpoint of the utilization of luminescence and thus has become a serious problem. The light take-out efficiency of the organic electroluminescent display is generally as low as about 20%.
In order to improve the light take-out efficiency, the provision of a lower refractive index layer on the electroluminescent layer in its light outgoing side has also been proposed, for example, in Advanced Materials 2001, 13, No. 15, August, P. 1149-1152, “Doubling Coupling-Out Efficiency in Organic Light-Emitting Devices Using a Thin Silica Aerogel Layer.” The claimed advantage of this construction is as follows. Light emitted from the luminescent layer is first radiated to the lower refractive index layer having a refractive index substantially equal to the refractive index of an air layer. By virtue of this, even after subsequent passage through a higher refractive index layer, total reflection does not take place, and, theoretically, the light can be entirely radiated to the air layer on the viewer side. In the above technique, a silica aerogel having a refractive index n of 1.01 to 1.10 is provided between the luminescent layer and the substrate, and the whole light emitted from the electroluminescent layer except for a single mode light constituting a part of the light emitted from the electroluminescent layer which is propagated through the electroluminescent layer, once enters the lower refractive index layer. Therefore, the occurrence of the total reflection in a subsequent stage at the interface between the higher refractive index layer and the lower refractive index layer can be prevented. The use of the silica aerogel, however, necessitates the provision of the step of dipping in water. Since water is a vital unfavorable factor in the organic electroluminescent layer which shortens the service life of the device, when commercialization is taken into consideration, the adoption of this technique using silica aerogel is difficult.
Further, Japanese Patent Laid-Open No. 74072/1999 discloses a method wherein a micro lens array is formed on a substrate to enhance light take-out efficiency. Even in this method, however, sealing of an insulating liquid having a lower refractive index into between a luminescent layer and a lens array poses problems of service life of an electroluminescent display, a complicate process and the like.
Japanese Patent Laid-Open No. 283751/1999 (Japanese Patent No. 2991183) discloses a method wherein the direction of advance of light, which has been radiated from the electroluminescent layer and causes total reflection, is deviated by a diffraction grating from the total reflection angle to improve light take-out efficiency. In this method, however, since wavelength dispersion occurs, particularly for short-wavelength display colors in a full-color display, a diffraction angle necessary for deviating the direction of advance of the light from the total reflection angle cannot be sometimes provided. Further, the dispersion of light caused by diffraction is likely to cause color blurring.
Japanese Patent Laid-Open No. 260559/2000 discloses a method wherein an assembly of optical fibers made of quartz glass or a polymer is cut into thin pieces to prepare a substrate, the whole cut surface thereof is covered with an organic material or an inorganic material, the surface roughness thereof is finished into 300 nm or less, and pixels are formed thereon. The claimed advantage of this device is that luminescence is incident to the inside of each fiber of the group of optical fibers, is then easily transmitted while being confined in each fiber by total reflection, and can be efficiently taken out, as transmitted light, to the outside of the system.
In the light propagated in this way, however, the angle of light outgoing surface to a normal is originally less than the total reflection angle. Therefore, in this method, to begin with, light rays in the total reflection region are leaked without being guided through the group of fibers. In the above publication, there is also a description on the use of tapered fibers. The intention of using the tapered fibers, however, is merely to perform magnified display of the pixels. Further, in the preparation of the substrate, bundling of a fiber array is carried out, and the bundle is then sliced to prepare the substrate. Therefore, difficulties are encountered in the preparation of the substrate.
Another problem involved in the organic electroluminescent device is a deterioration in visibility and contrast of a displayed image derived from external light. The deteriorated visibility and contrast of the displayed image are attributable to such a phenomenon that a large part of light introduced into the device from the outside thereof is reflected from a cathode formed of a metallic thin film, which reflects about 70% of visible light, and is radiated from the light outgoing surface of the device. A conventional method for solving this problem is to adopt such a construction that a polarizing plate and a quarter-wavelength plate are disposed in the front of an electroluminescent layer. In this construction, external light is first attenuated by the polarizing plate to not more than ½ and is further converted to a circularly polarized light by the quarter-wavelength plate. The circularly polarized light is converted by an electrode to a circularly polarized light in the opposite direction which is again converted by the quarter-wavelength plate to a linearly polarized light in a direction orthogonal to the incident light. The linearly polarized light is substantially entirely absorbed in the polarizing plate. In this method, however, disadvantageously, luminescence per se is also attenuated to about ½. Therefore, the efficiency of the display is sacrificed.